Summary The dominant arrhythmia post Fontan is atrial tachycardia, with complex atrial reentry circuits Their appearance is commonly coincident with hemodynamic deterioration and the arrhythmia typically contributes further to lower cardiac output, forming a vicious cycle that may be lethal Atrial fibrillation commonly alternates with other atrial tachycardias Current management is tending toward early conversion of the atriopulmonary Fontan to an extracardiac conduit, with concurrent atrial arrhythmia prevention surgery Following all types of Fontan surgeries, medical management is usually not sustainable for more than 2 to 3 years and, although amiodarone is the most effective medication, side effects are common Invasive electrophysiology studies and catheter ablation strategies can be very helpful and are recommended early in the absence of gross atrial dilation and to diagnose and treat concurrent congenital arrhythmia substrates including accessory pathways Hematologic and Immunologic Complications It is well recognized that the Fontan circulation presents a hypercoagulable state, with the incidence of thromboembolic events reported to vary between 8% and 20% of the population.127–129 This is likely an underestimation, in view of the occurrence of silent thromboembolism in this group.130 A prospective multicenter randomized controlled trial assessing several anticoagulant regimens reported a total thrombosis rate of 23% over 2 years.131 Only one-third of these events (8%) were symptomatic, with the remainder being detected during intensive surveillance as part of the study design Studies have described a peak thrombotic risk in the first year following Fontan completion, which plateaus over the next 3 to 4 years, before a second peak after 10 years.129,132 Moreover, the incidence of thromboembolic complications is higher in adults compared with children, suggesting an increase in risk with time that might relate to a gradual deterioration in vascular and liver function, exacerbated by a tendency to a more sedentary lifestyle in older and more debilitated patients The etiology of this prothrombotic state is multifactorial and involves all three factors of the Virchow triad, namely abnormal hemodynamics, a hypercoagulable state, and endothelial dysfunction (Fig 73.17) Potential factors include the low-velocity flow in the systemic veins, cavopulmonary connection and pulmonary arteries, atrial arrhythmias, persistent cyanosis related to right-toleft shunts, and an imbalance of intrinsic procoagulant and anticoagulant factors FIG 73.17 Factors contributing to prothrombotic state in a Fontan circulation Risk Factors for Thromboembolism Older age at the time of the Fontan operation is a risk factor for silent thromboembolism.130,133 Surprisingly, there appears to be a similar risk of thromboembolism among the different variants of the Fontan (Fig 73.18; Video 73.2).134–136 Although the presence of a right-to-left shunt is known to increase the risk of cerebral vascular embolization, the presence of a fenestration has not been associated with increased thromboembolic134 or stroke risk.137 This suggests that intrinsic hematologic abnormalities may be the most significant factors in the prothrombotic state in the Fontan circulation Compared with healthy controls, Fontan patients have reduced levels of procoagulant factors,